1. Field of the Invention
This invention relates to a method of inserting additional data into a compressed signal. For example, it relates to a method of inserting additional data into an audio or video frame.
2. Description of the Prior Art
Inserting additional data into a compressed signal, such as an audio or video frame, is well known. For example, the MPEG1 audio standard (ISO 11172-3, Information technology—Coding of moving and associated audio for digital storage media at up to about 1.5 Mbit/s) allows for the insertion of ‘ancillary data’ into a MPEG frame. This ‘ancillary data’ is inserted into a ‘ancillary data portion’ of the frame. By ‘ancillary data’ we refer to data not needed to decode the media data content in the frame (e.g. compressed audio or video data) according to the normal decoding rules or methods. ‘Media data’ refers to data that is needed to decode and generate uncompressed media from the frame (e.g. uncompressed audio or video). Media data is placed in the ‘media data portion’ of a frame; in MPEG 1, this comprises 32 sub-bands at varying scale factor levels. The ancillary data portion is used, for example, in DAB (Digital Audio Broadcasting to carry Programme Associated Data (PAD). It is also used to store information in MP3 data files using the ID3 format (see www.id3.org).
There are currently two principle means of inserting additional data into frames: both mechanisms insert the extra data into the ancillary data portion of a frame, as opposed to modifying the media data portion itself. The first mechanism involves reserving a known number of bytes of each MPEG audio frame for additional non-audio data. This involves an instruction to the MPEG encoder which ‘leaves blank’ the desired number of bytes; the ancillary data portion occupies this space. So, some audio quality is sacrificed for data insertion. This mechanism is supported by a number of MPEG encoders and is used in DAB (Digital Audio Broadcasting).
The second mechanism involves using VBR (Variable Bit Rate coding). In this scheme, an upper limit is specified for the size of the MPEG frame. The size of the encoded audio frame depends on the audio data being coded. If the data can be encoded in less than the upper limit, then it will be. The data insertion software would then claim any unused space below the upper limit for use as an auxiliary data portion. At the time of writing, most MPEG encoders do not support VBR coding.
Reference may also be made to a third (and quite unusual) technique: WO 00/07303 shows inserting extra data into the media data portion of a frame, rather than the auxiliary data portion of a frame. This is achieved by analysing the sub-bands in a frame and in effect adding data under the perceptible noise threshold of a sub-band.
The present invention relies on the detection of data frames that contain no information bearing data (e.g. audio silence or blank video), so it is also necessary to describe the prior art relevant to information loss detection. Being able to detect the presence or absence of information content in a compressed signal is a common requirement in many systems. For example, the compressed digital audio output from equipment used in broadcasting digital radio is usually monitored so that any silences lasting more than a set time period can be investigated in case they indicate a human error, or a software or equipment failure. More specifically, analysing a compressed signal for the presence or absence of information content may be used to detect when an audio service is no longer supplying audio to a DAB multiplexer, or in a video multiplexer to detect when one of the video channels suffers an audio or video loss.
The conventional approach to monitoring for losses of data in a compressed signal involves first fully decompressing the signal to a digital format (e.g. rendering it to PCM in the case of audio). It is the decompressed, digital signal which is then examined for silence (if audio) or lack of an image (if video) by comparing the decompressed digital signal against pre-set thresholds indicative of the presence or absence of information. If the compressed signal was taken from a digital source (e.g. a digital audio feed from a CD player), then this detection is relatively straightforward: the compressed signal is decompressed and the resultant PCM signals examined for events of zero amplitude: these correspond to the absence of any information content (e.g. silence in an audio frame), which may indicate a human error, or a software or equipment failure. If the signal was sourced from an analogue source prior to digitisation, then the procedure is more complex. An analogue source will never give true silence or lack of image. This analogue signal will pass through a digitising system and in most cases the resulting compressed signal will not be a ‘digital zero’ even when no genuine information is being carried. Hence, when decompressed, the resultant digital signal will also not be a digital zero even when no genuine information is being carried. In this case, the silence detecting system will have to apply some threshold based algorithm for deciding whether the signal contains data or not.
Although decompression is usually designed to be easier than compression, the decompression overhead is still significant
Whilst silence detection could be done at the digitising system, this may not be convenient for the broadcaster as the digitising system may be some distance from the multiplexer (and in fact could be owned and operated by a third party).